Why not use autosomal DNA

Lots of The Legal Genealogist‘s readers are helping out with ideas for the Great 2019 Research Caper: putting paid to the genealogical question of “who was the mother of Margaret (Battles) Shew?”

Margaret is my 3rd great grandmother. Her father, William Battles, was married twice, and it’s entirely possible that either wife could be Margaret’s mother. We have an mtDNA match to a documented descendant of wife #2, Ann Jacobs; we now need to rule out the possibility that wife #2 shared a common female ancestor — and thus mtDNA — with wife #1, Kiziah Wright.¹ Since we don’t know if Kiziah had any descendants, we need to find someone else who would share that same mtDNA signature — we need to find a documented female line descendant of Kiziah’s mother, whom we now know was Lucy (Jones) Wright Williford.²

One Wright descendant keeps trying, diligently, to help by providing more and more information about the Wrights, producing possible test candidates for autosomal DNA testing. And that cousin isn’t quite sure of the answer to the question: why not use autosomal DNA in this case?

It’s because it’s not going to help.

Here’s why:

• Autosomal DNA recombines — mixes randomly³ — in every generation, meaning that less and less DNA is passed from any one ancestor to any one descendant in every generation.⁴

• By the fourth cousin level, the odds of two cousins inheriting enough autosomal DNA in common to show up as a match drop to 50% or so, and by the fifth cousin level, the odds are up to 90% against having a detectable match even if the two are really fifth cousins.⁵ In other words, autosomal DNA punks out after just a few generations.

• Once autosomal DNA begins to punk out, it can’t disprove a relationship. If two people don’t match at an expected fourth or more distant cousin level, it doesn’t mean that they’re not fourth or fifth or sixth cousins. All you can say is that they didn’t inherit enough autosomal DNA in common to show up as a match.

As Blaine Bettinger explains, “In reality, everyone has two family trees. The first is a Genealogical Tree, which is every ancestor in history that had a child who had a child who had a child that ultimately led to you. Every decision made by every person in that tree contributed to who and what you are today. However, not every person in that tree contributed a segment of your DNA sequence…. As a result, we have a second family tree – a Genetic Tree – which is a tree that contains only those ancestors who contributed to our DNA.”⁶

So… here’s why I’m not looking for descendants of Frances and Lucy Wright to test against:

Anybody at my level would be — at best — a sixth cousin with a minute likelihood of having inherited enough autosomal DNA in common to be detected. Even if we test candidates at the suspected fifth cousin level — and I do have aunts and uncles to use as match candidates — getting a negative result against a descendant of a different child of Frances and Lucy (Jones) Wright doesn’t tell me we’re not related. It just tells me we didn’t get a match.

And here’s the real kicker: if we do get a match to a descendant of a different Wright child, it won’t tell me we’re related through the Wright line. I’d have to rule out every other possible shared ancestral line to be able to say, with assurance, that the shared DNA could only have come through the Wrights.

Stack that up against the possibility that I can disprove the possibility that we descend from Kiziah Wright by testing just one or two known female line descendants of Lucy (Jones) Wright, and there’s a pretty clear answer to the question we began with: why not use autosomal DNA in this case?

Cite/link to this post: Judy G. Russell, “Finding Margaret’s mother, part 5,” The Legal Genealogist (https://www.legalgenealogist.com/blog : posted 7 July 2019).

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